Recently, there are many mobile applications (Shorted as “app”) designed for mobile terminals such as mobile phones. When registration and login process to a server is necessary to operate the mobile application or use a specific service provided by the server, a username and a password are usually required to identify the user. For registration process, the user has to specify a username and a password. For login process, the user has to enter the specified username and the password. Then, after the entered username and password are authenticated, the user is allowed to operate the mobile application or use the specific service.
However, it is inconvenient to enter characters of the username and the password through mobile terminals (e.g. mobile phones) during registration and login process. Input errors possibly occur and operation of the mobile terminal is seriously affected.